PROPERTIES OF PURE SUBSTANCE

A substance that has a fixed chemical composition throughout is called a pure substance. Water, nitrogen, helium, and carbon dioxide, for example, are all pure substances. A pure substance does not have to be of a single chemical element or compound, however. A mixture of various chemical elements or compounds also qualifies as a pure substance as long as the mixture is homogeneous. Air, for example, is a mixture of several gases, but it is often considered to be a pure substance because it has a uniform chemical composition

PHASE CHANGE PROCESSES OF A PURE SUBSTANCE

There are many practical situations where two phases of a pure substance coexist in equilibrium. Water exists as a mixture of liquid and vapor in the boiler and the condenser of a steam power plant. The refrigerant turns fromliquid to vapor in the freezer of a refrigerator.

PROPERTY DIAGRAM FOR PHASE CHANGE PROCESSES

1. T-v DIAGRAM

T-v diagram of constant-pressure phase-change processes of a pure substance at various pressures

2. P-V DIAGRAM

P-v diagram of a pure substance.

P-v diagram of a substance that
contracts on freezing.

P-v diagram of a substance that
expands on freezing (such as water).

3. The P-T DIAGRAM

P-T diagram of pure substances.

THE P-V-T SURFACE

P-v-T surface of a substance that
contracts on freezing.

P-v-T surface of a substance that
expands on freezing (like water).

SATURATED LIQUID-VAPOUR MIXTURE

During a vaporization process, a substance exists as part liquid and part vapor. That is, it is a mixture of saturated liquid and saturated vapor. To analyze this mixture properly, we need to know the proportions of the liquid and vaporphases in the mixture. This is done by defining a new property called the quality x as the ratio of the mass of vapor to the total mass of the mixture:

Thus in a mixture of Vf and Vg the relative proportions are as follows: